1. Field of the Invention
The present invention relates to a noise filter array in which multiple filter elements are integrally provided with one another. Each filter element includes an LC parallel resonant circuit and an LC series resonant circuit each including a coil and a capacitor.
2. Description of the Related Art
Since various communication methods (for example, Global System for Mobile Communications (GSM), Digital Cellular System (DCS), and Personal Communications service (PCS)) are used for mobile phones, some conventional mobile phones use multiple communication bands. In such a case, in order to prevent the reception sensitivity within each communication band from being degraded, it is necessary to effectively suppress noise within each communication band.
In the noise suppression within communication bands near, for example, about 900 MHz and near about 1.8 GHz, noise filters are required to achieve attenuation over a wide range across the communication bands. In order to achieve such wide attenuation characteristics in filters, an inductance can be applied to grounded capacitors to define double-resonance filters.
In the related art, a double-resonance filter element shown in FIG. 10 is known (see, for example, Japanese Unexamined Patent Application Publication No. 9-266430). The double-resonance filter element in FIG. 10 includes an LC parallel resonant circuit PR in which a stray capacitor C1 is provided in parallel with a coil L1 provided on a signal line and an LC series resonant circuit SR in which a capacitor C2 is serially connected to a coil L2 between the signal line and the ground.
Although such a double-resonance filter element in the related art provides attenuation over a wide bandwidth, specifically, although such a double-resonance filter element can suppress the noise within each communication band near about 900 MHz or near about 1.8 GHz in the above example of the mobile phones, in practice, it is difficult to effectively suppress the noises within the communication bands using only one double-resonance filter element in a mobile phone using multiple communication bands.
Accordingly, multiple double-resonance filter elements corresponding to the multiple communication bands can be provided, and these filter elements can be integrally provided with one another to configure a noise filter array. The noise filter array in which the multiple double-resonance filter elements are integrally provided with one another has an array configuration as shown in FIG. 11. The array configuration in FIG. 11 is represented as an equivalent circuit. Four filter elements are provided in the exemplary noise filter array shown in FIG. 11.
In the array configuration shown in FIG. 11, since all of the portions corresponding to LC series resonant circuits surrounded by broken lines are grounded, the capacitors and the coils may be shared to provide a simple configuration. In such a case, for example, a noise filter array has a configuration shown in FIG. 12.
Specifically, in the noise filter array in FIG. 12, the signal-side electrodes of ground capacitors C12 to C42 including the LC series resonant circuits in the filter elements are provided for the respective filter elements while the capacitors C12 to C42 have a ground-side electrode commonly provided for the signal-side electrodes and one coil L0 is connected to the ground-side electrodes.
However, since only one coil L0 is commonly used in the LC series resonant circuits in the filter elements in the noise filter array having the configuration shown in FIG. 12, it is difficult to individually adjust the inductances of the LC series resonant circuits for each communication band. Consequently, there is a problem in that it is not possible to appropriately accommodate cases in which the filter elements are provided to suppress noises having different frequencies.